An increasingly large number of analytical analyses (i.e., assays) must be performed each day on many kinds of liquid samples, including, but not limited to, aqueous biological fluids such as blood, serum, urine, cerebral spinal fluid, and the like. To expedite the handling of these assays in the analytical laboratory, "dry chemistry" analytical elements have been proposed which are capable of analyzing for two or more analytes contained in a liquid sample. The utility of such an element is self evident. If a single analytical element can provide an assay for two or more analytes in a liquid sample, the number of test elements and/or liquid samples needed to conduct a complete analysis of the liquid can be substantially reduced. Moreover, if the assays can be conducted simultaneously, analysis time is likewise reduced.
Various "dry chemistry" analytical elements have been developed wherein a single test element is capable of multiple analyte detection. For example, U.S. Pat. No. 3,001,915 issued Sept. 26, 1961 and U.S. Pat. No. Re. 28,339, issued Feb. 18, 1975 each disclose a "dry chemistry" test element having the capability of detecting two or more analytes contained in a sample of test liquid. The dry test elements described in each of these patents provide multiple analyte detection by having a different interactive compositions for each of the analytes to be detected contained in a separate zone of the elements, the zones being longitudinally spaced along the length of the element. Each of the individual interactive compositions remains separated from one another upon application of the liquid test sample to the element and liquid contact among individual interactive compositions within the element is prevented. In use, a liquid sample applied to the element provides a measure of the concentration of each analyte in the sample by, for example, a separate color change occurring in each zone upon reaction of each interactive composition with the sample.
Another type of "dry chemistry" analytical element having multiple analyte detection capability is disclosed in West German patent-of-addition No. 2,347,111 published Mar. 27, 1975. This patent-of-addition discloses a single test strip having, in separate portions of the strip, individual precipitation reagent compositions, each of which is capable of analyzing for a different analyte of a liquid sample. In use, a liquid sample applied to this test strip simultaneously contacts each of the different portions of the test strip. A different precipitate thus forms in each portion of the test strip in response to the particular analyte for which the reagent contained in that portion is sensitive. The pattern of light scattering produced by the attenuation of a light beam directed at the precipitate formation in each portion of the test strip is then used as a measure to determine the presence and/or concentration of the individual analytes contained in the liquid sample. Although each of the different reagent compositions of the test strip disclosed in West German patent-of-addition No. 2,347,111 is located in a distinct portion of the strip, typically with a physical gap therebetween, liquid contact between these individual reagent compositions apparently can occur.
Liquid contact occurring among the individual reagent compositions of a test element for multiple analyte detection can present a real problem. One is faced with the problem of preventing detection signal interference, i.e., preventing interference between a detectable signal generated by one reagent composition upon interaction with one of the analytes and a separate signal produced upon interaction of a second reagent composition with one of the other analytes.
This interference problem is particularly acute where analyte presence and/or concentration is detected by conventional radiometric detection techniques. These techniques rely upon changes in the absorption spectrum of a reference light beam transmitted through or reflected from the element or upon changes in the emission spectrum of the element in response to such a reference light beam. This is because one must distinguish between possible overlapping absorption or emission bands if the interactive product(s) of one analyte reaction come into fluid contact with the interactive product(s) of another analyte.
To alleviate this interference problem, the analytical elements described in the aforementioned U.S. Pat. Nos. 3,001,915 and Re. 28,339 employ special barrier means or compositions to completely prevent the occurrence of fluid contact among individual interactive compositions. In West German patent-of-addition No. 2,347,111 the problem is apparently prevented or at least reduced by determining analyte concentration as a function of a light scattering pattern arising from precipitate formation, rather than using a conventional colorimetric or fluorometric radiometric detection system relying upon a change in the absorption or emission spectrum of the element to measure analyte concentration.
An element for analysis of multiple analytes also presents the further difficulty of obtaining maximum sensitivity for two or more analytes, one of which may be present at an abnormally low concentration and one other of which at an abnormally high concentration.
For example, in certain pathological conditions such as jaundice, diagnosis often depends upon being able to determine the concentration of one serum analyte, e.g., cholesterol, which may be severely depressed and the concentration of another serum analyte, e.g., bilirubin, which may be highly elevated. When these two analytes are evaluated at these abnormal concentration levels by conventional absorption techniques, however, one obtains a strong signal corresponding to the elevated concentration of the one analyte (which generally leads to high sensitivity), and, unfortunately, a weak signal corresponding to the depressed concentration of the other analyte (which generally leads to low sensitivity).
A single, dry chemistry analytical element capable of multiple analyte detection while retaining the simplicity of conventional absorption or emission detection techniques would be of substantial benefit to the art. Such an element would be particularly desirable if it did not require special barrier means to prevent liquid contact among individual interactive compositions contained in the element (thereby simplifying element manufacture and construction) and could minimize the potential interference problem presented by multiple analytes as well as reduce the difficulty of obtaining good sensitivity for multiple analytes, even though one analyte is present at an abnormally low concentration while another is present at an elevated concentration.